Limbal stem cells have been identified as slow-cycling, label-retaining cells in mice. Limbal stem cells express the nuclear transcription factor ΔNp63α in humans and lack expression of corneal epithelial differentiation markers such as KRT12 [2,8] (FIG. 1A). Limbal stem cells generate transient amplifying cells, which express the eye development master regulator PAX6 [9] and, during corneal development and regeneration, migrate out of the limbus to give rise to the KRT12(+) central corneal epithelium [10].